In recent years, most automotive steering systems generally use a rack and pinion mechanism as illustrated in FIG. 10. As well known in the art, a rotational movement of a pinion 4, which is connected, via a connecting shaft 3 including a universal joint, to a steering shaft 2 integrally carrying a steering wheel 1, is converted into a linear movement of a rack 5 meshing with the pinion 4, and the linear movement of the rack 5 is in turn converted into a steering movement of steerable wheels 8 via tie rods 6 and knuckle arms 7.
According to such a conventional steering system, the steering angle of the steerable wheels has a linear steering property, or, in other words, changes substantially linearly with the rotational angle of the steering wheel, but, in terms of the maneuverability of the vehicle, the rotational angle of the steering wheel for achieving a maximum steering angle of the steerable wheels is desired to be relatively small.
For instance, when the steering angle property is set as indicated by a chain-dot line (a) of FIG. 11, the rotational angle of the steering wheel is kept small, and a favorable maneuverability can be achieved in a low speed range. However, in a high speed range, the behavior of the vehicle becomes excessively sensitive to the steering angle of the steering wheel, and the vehicle operator is required to be careful not to excessively turn the steering wheel. This is because the relation between the steering angle of the steering wheel and the behavior of the vehicle is not fixed, or, more specifically, the yaw response of the vehicle for a given steering angle of the steering wheel depends on the vehicle speed. For instance, as the vehicle speed increases, the yaw response of the vehicle tends to become increasingly more sensitive.
For this reason, the steering property of a vehicle is generally determined as a compromise so that the maximum steering angle of the road wheels may be achieved when the steering wheel is turned by 1.5 turns in either direction from its neutral position, and the stability of the vehicle in a high speed range may be ensured (refer to the fine solid line (b) of FIG. 11) while achieving a tolerable level of maneuverability in a low speed range.
However, when the actual steering angle of the steering wheel is measured in relation with the vehicle speed, it can be found that the steering angle at the steering wheel tends to be excessive in a low speed range as indicated by the solid fine line (c) of FIG. 12.